The present invention relates to an optical device which has on its surface a number of structures with a higher portion or a lower portion being arranged at a fine pitch equal to or shorter than the wavelength of visible light, a method for producing a master for use in producing an optical device, and a photoelectric conversion apparatus.
There have been optical devices using a translucent base material such as glass or a plastic, and having a surface treated to suppress the surface reflection by light. As a surface treatment of this type, there is a method in which an optical device has a fine, dense uneven (moth eye) shaped surface formed thereon (see, for example, “Phototechnology Contact”, Vol. 43, No. 11 (2005), 630-637).
Typically, in an optical device having a surface with periodic unevenness shaped thereon, when light passes through the uneven surface of the optical device, diffraction of light occurs, and the straight component of the transmitted light is considerably reduced. However, when the pitch of the uneven shape is shorter than the wavelength of light which passes through, no diffraction occurs, and thus, effective anti-reflection properties can be obtained.
An optical device having the surface structure is shown in FIG. 29 (see, for example, Japanese Patent Application Publication No. JP 2003-294910). An optical device 101 has a configuration in which a number of conical and higher structures 103 are arranged on the surface of a base 102 at a fine pitch equal to or shorter than the wavelength of light (visible light). In the optical device 101 having the above surface structure, a gentle change of the refractive index is caused at an interface between the slope portions of structures 103 and the air layer to effectively prevent the reflection of the incident light from the surface of the base 102. The form of the structures 103 is not limited to the higher form and a similar effect can be obtained, even when the structures are formed of lower portions.
Further, with respect to the structures 103, a variety of cross-sectional forms and arrangements are proposed. For example, in the optical device 101 shown in FIG. 28, each of the structures 103 having the shown form is a lattice unit arranged to form a square lattice pattern. On the other hand, for example, in Japanese Patent Application Publication No. JP 2004-317922, an example is disclosed in which, as shown in FIG. 30, structures 104 are arranged to form a regular hexagonal lattice pattern. Further, in Japanese Patent Application Publication No. JP 2004-317922, an example is shown in which structures have conical forms.
By the way, with respect to the production of the optical devices, a replica substrate is formed from a master prototype (master) having a surface microstructure constituting each structure, and further a molding die is formed from the replica substrate. By the above, it is expected that the optical devices are mass-produced at low cost by molding. Specifically, an ultraviolet-cured replica substrate is formed by using a single master prototype, and a molding die is formed from the replica substrate by a plating technique, and whereby optical devices formed of, e.g., a polycarbonate resin can be mass-produced by injection molding using the molding die.
The master prototype is produced as follows. A resist is applied on a substrate, and subjected to exposure and development processing to form a resist pattern, followed by dry etching using this resist pattern as a mask. Then, the resist pattern (or mask pattern) is removed to form an uneven surface structure in which structures with a higher portion or a lower portion are arranged at a fine pitch on the surface of the substrate. It is noted, as a substrate material forming the master prototype, an inorganic material, such as quartz, or the like may be used.
For the production of the master prototype, highly precise shape same level with that of the fine resist pattern formed on the surface of the substrate. As a technique for forming a pattern at a fine pitch equal to or shorter than the wavelength of visible light with high precision, for example, electron beam exposure is known.
As a moth eye structure formed by the electron beam exposure, fine tent-form moth eye structures (pitch: about 300 nm; depth: about 400 nm) shown in FIG. 31 are disclosed (see “Mold die master for anti-reflective structure (moth eye) free of the wavelength dependency”, NTT Advanced Technology Corporation, (online), (searched on Aug. 20, 2007), Internet). The moth eye structures seem to be produced by a method in which an uneven pattern is formed by electron beam recording performed to a photoresist on a Si (silicon) substrate, and the resultant substrate is subjected to anisotropic etching for the Si substrate surface using the uneven photoresist pattern as a mask. The moth eye structures is formed to be a hexagonal lattice as shown in FIG. 32, and whereby anti-reflection effects (reflectance: 1% or less) with very high functions in the wavelength range of visible light is obtained. FIG. 33 shows the wavelength dependency characteristics of the reflectance with respect to the Si master.
However, the electron beam exposure has a disadvantage in that operation hours are long, and hence it is not suitable for the industrial production. For example, in the electron beam exposure using an electron beam at 100 pA, which is used in drawing the finest pattern, for a resist which requires dose amounts of several tens mC/cm2, such as Calixarene, even if the exposure is continued for 24 hours, an area of a square having one side of 200 μm cannot be filled. Further, the electron beam exposure for an area of a 2.5-inch small-size display (50.8 mm×38.1 mm) of currently typically used mobile phones requires around 20 days.
Therefore, a technique for producing a master prototype at low cost in short hours is desired. To meet this demand, a technique for producing the master prototype by laser exposure is proposed. Specifically, various techniques for producing the master prototype utilizing a mastering technique of optical disc are considered.
For example, an optical device having a nanometer-size microstructure (low-reflective moth eye structure) formed on a disc-form Si substrate having a diameter of 12 cm by utilizing the mastering technique of optical disc is disclosed (see “Development of a Desktop Apparatus Enabling Nanometer-scale Fine Processing”, National Institute of Advanced Industrial Science and Technology, (online), (searched on Aug. 20, 2007)).
This document indicates that according to this method, a dot pattern of 50 nm, which corresponds to one sixth or less of the size of a laser beam spot, can be formed at a speed of 6,000,000 dots/s by irradiation with a laser beam at a pulse frequency of 60 MHz while rotating the substrate at a speed of 6 m/s. FIG. 34 shows an example of forming a nanodot pattern of this optical device.
Further, a method is disclosed in which six spots are formed at each apex of a regular hexagonal shape in a photoresist layer on a surface of a glass master by parallelly separating a laser beam into six laser beams using a phase diffraction grating having regular triangle patterns and a sawtooth diffraction grating having six regions, and causing the six laser beams converged into a single spot by an objective lens to be interfered each other (for example, Japanese Patent Application Publication No. JP 2003-131390).
However, an optical device produced by the technologies has poor wavelength dependency characteristics of the reflectance, and cannot realize a low reflectance of 1% or less, and therefore it is not suitable to be put into practical use as an anti-reflection structure. The reason for this is presumed that the optical device has a low density (50% or less) of the nanodot pattern (opening ratio) and a large Fresnel reflection in the plane region of the pattern-non-formed portion. FIG. 35 shows reflection properties of the optical device shown in FIG. 34.
The present invention is made in view of the issues and aims to provide an optical device that has a high productivity and an excellent anti-reflection property, a method for producing a master for use in producing an optical device and a photoelectric conversion apparatus.